1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
In internal combustion engines provided with a three-way catalytic converter in the exhaust system for the purificaiton of exhaust gas, a feedback control is generally executed for controlling the air/fuel ratio in accordance with the composition of exhaust gas and the operating conditions of the engine. This is because an optimum operation of the three-way catalytic converter is attained when the air/fuel ratio of mixture is around a stoichiometric value (14.7:1 for example). An air intake side secondary air supply system for feedback control is an example of the system having air/fuel ratio control operation of this type, in which a secondary air passage leading to a portion of an intake air passage downstream of the throttle valve is provided and the amount of a secondary air flowing therethrough is controlled for performing the air/fuel ratio control.
An air intake side secondary air supply system of this type is disclosed in Japanese patent application laid open No. 59-90753. In this system, an air control valve whose opening degree varies depending on a magnitude of pressure within its pressure chamber is provided in the secondary air supply passage leading to the portion of the intake manifold downstream of the throttle valve. An air/fuel ratio of the mixture supplied to the engine is detected from a concentration of a component of the exhaust gas, to produce an air/fuel ratio signal indicating a result of the detection of air/fuel ratio. In accordance with the air/fuel ratio signal, an integral control of air/fuel ratio is performed by supplying either one of a first control pressure for gradually opening the air control valve to increase a sectional area of the flow through the valve and a second control pressure for gradually closing the air control valve to decrease the sectional area, into the pressure chamber.
In such an air intake side secondary air supply system, the first and second control pressures are in turn supplied to the pressure chamber of the air control valve in response to a state of the air/fuel ratio signal, through a three-way solenoid valve having a switching operation for example, and a pressure supply passage. For performing the integral control operation, the first and second control pressures are supplied to the three-way valve through an orifice provided in the pressure supply passage. Also, if a surge tank is provided in the above mentioned pressure supply passage for suppressing the pulsation of the pressure, the capacity of the surge tank has an influence upon a time constant of the integration operation. The speed of increase or decrease of the opening degree of the air control valve is maintained constant while it is depending on the diameter of the orifice and the capacity of the surge tank. In this way, the air/fuel ratio of the mixture supplied to the engine increases or decreases gradually towards the stoichiometric air/fuel ratio by the increase or decrease of the amount of the secondary air supplied to the engine.
Further, in the air intake side secondary air supply system, an open loop control mode is also provided in which the feedback control of the air/fuel ratio is stopped to enrich the air/fuel ratio so as to ensure a stable combustion during a low load operation of the engine such as a decelerating state of the engine. Such an open loop control is also selected during a high load condition, an accelerating state of the engine for example, in order to provide a good driveability.
However, in such a case, a delay of closure of the air control valve generally occurs upon transition from the closed loop mode operation to the open loop mode operation under the low load or the high load condition even if the supply of the first control pressure into the pressure chamber of the air control valve is stopped immediately and the supply of the second control pressure is started subsequently. This is because of a characteristic of the integration operation such that a time period is required in order that the pressure in the pressure chamber reaches the level of the second control pressure to close the air control valve. Therefore the enrichment of the air/fuel ratio is delayed upon transition from the closed loop mode to the open loop mode, to cause an increase of noxious unburned components of the exhaust gas and a deterioration of the driveability.